There have conventionally been proposed, e.g., a wire dot method, a thermal method, a thermal transfer method, and an inkjet method as printing methods of printing apparatuses which print on a printing medium such as paper or a plastic sheet. Of these printing apparatuses, a printing apparatus (inkjet printing apparatus) which adopts the inkjet method of discharging ink from a discharge orifice to print on a printing medium achieves quiet non impact printing and can print at high density and high speed.
Recently, printing at higher speeds and higher densities are required. To meet this demand, a printhead (to be referred to as an inkjet printhead hereinafter) mounted in an inkjet printing apparatus generally has many discharge orifices for discharging ink. Some discharge methods for the inkjet printhead utilize, as ink discharge energy, abrupt ink bubbling upon driving a heating element (to be also referred to as a nozzle heater hereinafter) such as an electrothermal transducer arranged in the discharge orifice. Some discharge methods utilize contraction upon driving a piezoelectric element attached to a nozzle.
Regardless of the employed method, discharge becomes unstable due to pressure interference (crosstalk) between adjacent nozzles when all printing elements are concurrently driven in printing. In addition, a voltage drop by power loss on a common power line becomes large near the printhead owing to a large current. As the number of concurrently driven nozzles increases, the driving voltage applied to a nozzle heater drops much more, and printing stability is impaired. Further, the design of a compact, low cost apparatus is limited such that a power supply sufficient to resist an instantaneous large current is required. This problem is solved by dividing all nozzles into a plurality of blocks each having several to several ten nozzles in an inkjet printhead and sequentially time divisionally driving nozzles in the respective blocks. This driving method is called time divisional driving or block divisional driving.
FIG. 9 is a block diagram showing a general configuration of the driving circuit of an inkjet printhead (to be referred to as a printhead hereinafter) using the time-divisional driving method.
In FIG. 9, M printing elements R01 to RM are commonly connected to a driving voltage VH at one end, and to an M-bit driver 301 at the other end. The M-bit driver 301 receives AND signals of an output signal from an M-bit latch 302 and block enable selection signals (BE1 to BEN) of N bits. The M-bit latch 302 receives signals of M bits output from an M-bit register 303. When a latch signal (LAT) is supplied to the latch circuit, the M-bit latch 302 latches (records and holds) M-bit data stored in the M-bit register 303. The M-bit shift register 303 is a circuit which aligns and stores image data in correspondence with printing elements. The shift register receives image data which is sent via a signal line S_IN in synchronism with an image data transfer clock (SCLK).
In the driving circuit having the above configuration, time-divisional driving signals are sequentially input as the block enable selection signals (BE1 to BEN) to time-divisionally drive N printing elements in respective blocks. That is, a plurality of printing elements of the printhead are divided into a plurality of blocks and time-divisional driven to print.
When the number of time-divisionally driven blocks is large, it is known to attach a block enable selection decoder in order to decrease the number of input signals.
When the number of printing elements in a block is set to N for M nozzles, a signal output from the block enable selection decoder can be formed from (MIN) bits. The relationship between the MIN value and the number (X) of terminals of the block enable selection decoder is
Time-Divisional Count (Block Count) NN=M/N=2X The number of enable terminals can be decreased from M/N to X.
However, when the printhead having printing elements arranged on the same line is time-divisional driven block by block, the printing position shifts between blocks because the carriage which supports the printhead moves in the scanning direction. The shift in printing position between blocks becomes large in a printhead which has many blocks and is equipped with the above-mentioned block enable selection decoder.
In order to solve this problem, for example, Japanese Patent Publication For Opposition No. 3-208656 proposes a sequential distribution driving method which prevents the printing shift between blocks by using a printhead configured by inclining a printing element array from the carriage moving direction.
In general, however, the same printhead is driven at various driving frequencies in accordance with the printing mode or a printing apparatus on which the printhead is mounted. For this reason, in a printhead which has many blocks and is equipped with the block enable selection decoder, the highest driving frequency must be assumed to determine the number of blocks. In this case, the method disclosed in Japanese Patent Publication For Opposition No. 3-208656 cannot be used.
As a method of preventing a shift in printing position even in this case, Japanese Patent Publication Laid Open No. 7-323612 discloses a method of divisionally driving printing elements in correspondence with the moving speed when the printhead is scanned.
Japanese Patent Publication Laid Open No. 2001-347663 proposes a printhead in which printing elements are arranged by shifting their positions in consideration of the printing position by time-divisional driving.
In the printing field, a technique of performing digital-halftoning (pseudo-halftoning), i.e., forming a unit matrix (image processing control unit of M×N pixels) from dots in order to implement high-quality printing is well known. In electrophotography, clustered-dot digital-halftoning of fatting dots as the density increases from the center of a matrix used for printing is known particularly as a means for improving color reproducibility of a color image (see, e.g., Japanese Patent No. 2553045). Also in inkjet printing, there is known a technique of improving the image quality by performing digital-halftoning control in a halftone or clustered-dot unit matrix. Examples of this technique are disclosed in Japanese Patent Publication Laid Open Nos. 7-232434, 11-5298, 2000-118007, 2000-198237, 2000-350026, and 2002-29097.
However, these prior art techniques suffer the following problems when printing is done by time divisional driving in digital halftoning by the above mentioned unit matrix.
FIG. 10 is a schematic view showing the relationship between the nozzle array of a printhead, a driving signal for each nozzle, and a dot which is discharged from each nozzle and attached onto a printing medium.
An example shown in FIG. 10 is 1-pass printing in a serial inkjet printing apparatus which prints by reciprocating a carriage which supports a printhead.
As shown in a of FIG. 10, a nozzle array 500 of the printhead is divided into 86, first to 86th sections each having six nozzles from the top of FIG. 10. Each of six nozzles in each section belongs to one of six driving blocks, and the nozzles of the respective blocks are time-divisionally driven in printing. That is, nozzles in the same block are concurrently driven.
In the example shown in FIG. 10, all nozzles are periodically assigned to driving blocks such that the first, seventh, 13th, 19th, . . . nozzles of the nozzle array 500 are assigned to the first driving block, and the second, eighth, 14th, 20th, . . . nozzles are assigned to the second driving block. The first to sixth driving blocks are sequentially driven in ascending order by a pulse-like driving signal 300 shown in b of FIG. 10. As shown in c of FIG. 10, dots 100 are formed from the nozzles onto a printing medium in correspondence with the driving signal.
At this time, the unit matrix size is 8×8. As is apparent from c of FIG. 10 showing the attaching position of an ink droplet, the shape of a dot cluster which forms a unit matrix changes depending on the printing position due to the relationship between time-divisional driving and the unit section size.
The shape difference is generated because the section size is “6” and the unit matrix size in the nozzle array direction is “8” in the example shown in FIG. 10. More specifically, patterns of different shapes each in a predetermined period shorter than the period of the unit matrix in the nozzle array direction are repetitively formed in a predetermined period of 24 pixels which is the least common multiple of “6” and “8”. In this manner, the shape of a dot cluster in each unit matrix periodically changes owing to the relationship between the unit matrix size and the unit section size of time-divisional driving. The periodical change appears as periodical density unevenness to the eye, degrading the image quality.
Since the shape of each unit matrix changes depending on the printing position, ink droplets which form adjacent unit matrices come into contact with each other on a printing medium particularly in high speed printing to degrade the image quality with a higher probability, in comparison with a case wherein dot clusters of the same shape are formed.
For this reason, it is desired to form dot clusters of the same shape in unit matrices regardless of the image printing position.
This problem occurs not only in 1-pass printing by the serial printing apparatus. For example, even multi-pass printing or a printing apparatus which supports a full-line type printhead may pose the same problem depending on the relationship between the unit matrix size and the unit section size of time-divisional driving, degrading the image quality.